The invention relates to an expandable OFDM method based on sub carrier without changed width. Wherein, it comprises that preprocessing step that coding and modulating the original data; serial-parallel conversation step that converts the original data from serial to parallel; sub-carrier distribution that based on user level divides relative sub-carrier region; power compensation step that compensates the transmission power to the user terminal with low level; power and sub-carrier distribution that distributing algorism to the transmission power and sub carrier; IFFT processing step that orthogonally crossing all sub carriers of whole band width, and converting it from frequency domain to time domain; insertion protective slit step that inserting protective slim to avoid serial interference between marks; and parallel-serial convert step that recovering the serial on parallel data. The invention can optimize the utilization of wireless resource without change the efficiency of OFDM mark.